Method and apparatus for microcellular polypropylene extrusion, and polypropylene articles produced thereby

ABSTRACT

A polymeric foam article is comprised mainly of homopolymeric polypropylene, or unimodal polypropylene. The polymeric article can be microcellular, and can be formed into a drinking straw.

FIELD OF THE INVENTION

[0001] The present invention relates generally to polymeric foamprocessing, and more particularly to a continuous microcellular polymerextrusion system and method that allows extrusion of microcellular,standard-grade polypropylene. Polymer foam straws also are produced.

BACKGROUND OF THE INVENTION

[0002] Foamed polymeric materials are well known, and typically areproduced by introducing a physical blowing agent into a molten polymericstream, mixing the blowing agent with the polymer, and extruding themixture into the atmosphere while shaping the mixture. Exposure toatmospheric conditions causes the blowing agent to gasify, therebyforming cells in the polymer. Under some conditions the cells can bemade to remain isolated, and a closed-cell foamed material results.Under other, typically more violent foaming conditions, the cellsrupture or become interconnected and an open-cell material results. Asan alternative to a physical blowing agent, a chemical blowing agent canbe used which undergoes chemical decomposition in the polymer materialcausing formation of a gas.

[0003] Foamed polyolefins are known. Of these, polyethylene is preferredbecause of ease of foaming control. While foams including polypropylenecomponents are known, in most cases such foams include significantproportion of additives that add controlability to the foaming process.

[0004] U.S. Pat. No. 4,940,736 (Alteeping) describes a foamed productmade by foaming a composition including a major proportion of a lowviscosity polypropylene having a melt viscosity of less than 2×10³ poiseand a minor proportion of a high viscosity polypropylene having a meltviscosity of greater than 2.5×10³ poise. Alteeping mentions thatpreviously-proposed procedures for foaming polypropylene had sufferedfrom severe disadvantages limiting their commercial application, notingspecifically the following: U.S. Pat. No. 4,352,892 (Firma CarlFreudenberg), which discloses foaming a composition includingcrystalline polypropylene and a further component selected frompolybutadiene, ethylene vinyl acetate copolymer, and ethylene-propyleneterpolymer rubbers; U.S. Pat. No. 4,442,232 (Firma Carl Freudenberg)which discloses foams comprising crystalline polypropylene andpolybutadiene that are cross-linked; U.S. Pat. No. 4,298,706(Karengafuchi Dagaku Koguyo KK) which discloses foams of compositionscomprising of polypropylene and polybutadiene kneaded together; U.S.Pat. No. 3,846,349 (Sumitomo Chemical Co.) which describes foam producedfrom a three-component mixture of crystalline polypropylene,non-crystalline propylene, and low density polyethylene; and U.S. Pat.No. 3,607,796 (Grunzweig and Hartmann AG) which describes a process forproducing foam from a composition comprising high and low molecularweight polypropylene.

[0005] U.S. Pat. No. 5,180,751 (Park) describe polypropylene foam madeof polypropylene resins having a z-average molecular weight above 1×10⁶and a z-average molecular weight/weight average molecular weight ratioabove 3.0. Park states that unacceptable foam sheets show a unimodalmolecular weight distribution, while resins which yield acceptable foamsheets show a bimodal molecular weight distribution.

[0006] U.S. Pat. No. 4,832,770 (Nojiri) describes a method ofmanufacturing a foamed polypropylene resin from a mixture of 80 to 20weight percent of a crystalline polypropylene-ethylene block copolymercontaining 20 weight percent or less of ethylene and having a melt indexof two or less and 20 to 80 weight percent of a crystallinepolypropylene-ethylene block or random copolymer containing 5 weightpercent or less of ethylene and having a melt index of 6 to 20 or apolypropylene homopolymer having a melt index of 6 to 20.

[0007] One class polymer foams that can offer a variety of advantageouscharacteristics such as uniform cell size and structure, the appearanceof solid plastic, etc. are microcellular foams. U.S. Pat. No. 4,473,665(Martini-Vvedensky, et al.; Sep. 25, 1984) describes a process formaking foamed polymer having cells less than about 100 microns indiameter. In the technique of Martini-Vvedensky, et al., a materialprecursor is saturated with a blowing agent, the material is placedunder high pressure, and the pressure is rapidly dropped to nucleate theblowing agent and to allow the formation of cells. The material then isfrozen rapidly to maintain a desired distribution of microcells.

[0008] U.S. Pat. No. 5,158,986 (Cha, et al.; Oct. 27, 1992) describesformation of microcellular polymeric material using a supercriticalfluid as a blowing agent. In a batch process of Cha, et al., a plasticarticle is submerged at pressure in supercritical fluid for a period oftime, and then quickly returned to ambient conditions creating asolubility change and nucleation. In a continuous process, a polymericsheet is extruded, then run through rollers in a container ofsupercritical fluid at high pressure, and then exposed quickly toambient conditions. In another continuous process, a supercriticalfluid-saturated molten polymeric stream is established. The stream israpidly heated, and the resulting thermodynamic instability (solubilitychange) creates sites of nucleation, while the system is maintainedunder pressure preventing significant growth of cells. The material thenis injected into a mold cavity where pressure is reduced and cells areallowed to grow.

[0009] While polymer foams containing polypropylene exist, it would beadvantageous, in terms of added simplicity and reduced cost, to be ableto produce high-quality foams including polypropylene without the needfor significant amounts of foam-controlability additives or otherco-polymerized or blended polymer components. It is an object of thepresent invention to provide such articles.

SUMMARY OF THE INVENTION

[0010] The present invention provides methods and systems for producingpolymeric polypropylene foam which can be microcellular material, andarticles produced thereby.

[0011] In one aspect the invention provides extrusion systems.Specifically, an extruder is provided that has an inlet for receiving aprecursor of a foamed polypropylene material at an inlet end thereof,and an outlet at an outlet end thereof for releasing foamedpolypropylene material from the extruder. An enclosed passagewayconnects the inlet with the outlet. The passageway is constructed andarranged to contain a product of the mixture of a blowing agent whichcan be a supercritical fluid, in particular supercritical carbondioxide, with molten polypropylene material to be foamed within thepassageway and to maintain the product within the passageway. Theproduct can be maintained within the passageway above the criticaltemperature and pressure of the supercritical fluid. A nucleator isassociated with the passageway and is capable of nucleating the productin the passageway in the absence of auxiliary nucleating agent, althoughnucleating agent can be used. An orifice is provided between the inletand the outlet and is fluidly connectable to a source of blowing agentwhich can be supercritical fluid. The system receives polypropylenehaving a unimodal molecular weight distribution or other polypropylenedescribed below in connection with the articles of the invention.

[0012] In another aspect the invention provides a method. One methodinvolves providing a polypropylene material to be foamed, selected amongpolypropylene described below with respect to articles of the invention,and mixing a blowing agent into the material to create a mixture. Ahomogeneous single-phase solution is created from the mixture that has auniform concentration of blowing agent distributed therein. Thehomogeneous single-phase solution is nucleated, and then essentiallyimmediately thereafter shaped to create a shaped extrudate. The shapedextrudate can be released into ambient conditions essentiallyimmediately after shaping. Nucleation can take place by passing thesingle-phase solution through a constriction creating a rapid pressuredrop due to friction.

[0013] In another aspect the invention provides articles. One article isa foamed microcellular polypropylene article having an average cell sizeof less than about 100 microns.

[0014] In another embodiment an article of the invention is a foamedpolymeric article including at least about 80% by weight polypropylenehaving a unimodal molecular weight distribution.

[0015] In another embodiment the invention provides an article includingfoamed polymeric material including at least about 80% by weighthomopolymeric polypropylene of viscosity of at least about 2.5×10³poise.

[0016] In another embodiment the invention provides a foam, polymericdrinking straw.

[0017] The invention also provides a foamed polymeric tubular articlehaving a length-to-diameter ratio of at least about 10 and a wallthickness of no more than about 1.0 millimeters.

[0018] The invention also provides a foamed polymeric tubular articlehaving a diameter-to-thickness ratio of from about 9:1 to about 50:1.

[0019] Other advantages, novel features, and objects of the inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings,which are schematic and which are not intended to be drawn to scale. Inthe figures, each identical or nearly identical component that isillustrated in various figures is represented by a single numeral. Forpurposes of clarity, not every component is labeled in every figure, noris every component of each embodiment of the invention shown whereillustration is not necessary to allow those of ordinary skill in theart to understand the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a schematic illustration of a polymeric extrusion systemthat can be used in accordance with one embodiment of the invention;

[0021]FIG. 2 is an alternative polymeric extrusion system that can beused in accordance with the invention;

[0022]FIG. 3 is a photocopy of an SEM image of microcellularhomopolymeric polypropylene foam of the invention;

[0023]FIG. 4 is a photocopy of an SEM image of microcellularpolypropylene extrudate of the invention;

[0024]FIG. 5 is a photocopy of an SEM image of microcellularhomopolymeric polypropylene of the invention;

[0025]FIG. 6 is a photocopy of an SEM image of talc-filled tubularmicrocellular polypropylene of the invention;

[0026]FIG. 7 is a photocopy of an SEM image of talc and colorconcentrate-filled tubular microcellular polypropylene of the invention;

[0027]FIG. 8 is a photocopy of an SEM image of medium-densitymicrocellular fractional melt flow polypropylene of the invention; and

[0028]FIG. 9 is a photocopy of an SEM image of medium-density sheetmicrocellular polypropylene of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0029] Commonly-owned, co-pending U.S. patent application Ser. No.08/777,709 “Method and Apparatus for Microcellular Polymer Extrusion”,filed Dec. 20, 1996 and commonly-owned, co-pending International patentapplication serial no. PCT/US97/15088, filed Aug. 26, 1997 areincorporated herein by reference.

[0030] The various embodiments and aspects of the invention will bebetter understood from the following definitions. As used herein,“nucleation” defines a process by which a homogeneous, single-phasesolution of polymeric material, in which is dissolved molecules of aspecies that is a gas under ambient conditions, undergoes formations ofclusters of molecules of the species that define “nucleation sites”,from which cells will grow. That is, “nucleation” means a change from ahomogeneous, single-phase solution to a mixture in which sites ofaggregation of at least several molecules of blowing agent are formed.Nucleation defines that transitory state when gas, in solution in apolymer melt, comes out of solution to form a suspension of bubbleswithin the polymer melt. Generally this transition state is forced tooccur by changing the solubility of the polymer melt from a state ofsufficient solubility to contain a certain quantity of gas in solutionto a state of insufficient solubility to contain that same quantity ofgas in solution. Nucleation can be effected by subjecting thehomogeneous, single-phase solution to rapid thermodynamic instability,such as rapid temperature change, rapid pressure drop, or both. Rapidpressure drop can be created using a nucleating pathway, defined below.Rapid temperature change can be created using a heated portion of anextruder, a hot glycerine bath, or the like. A “nucleating agent” is adispersed agent, such as talc or other filler particles, added to apolymer and able to promote formation of nucleation sites from asingle-phase, homogeneous solution. Thus “nucleation sites” do notdefine locations, within a polymer, at which nucleating agent particlesreside. “Nucleated” refers to a state of a fluid polymeric material thathad contained a single-phase, homogeneous solution including a dissolvedspecies that is a gas under ambient conditions, following an event(typically thermodynamic instability) leading to the formation ofnucleation sites. “Non-nucleated” refers to a state defined by ahomogeneous, single-phase solution of polymeric material and dissolvedspecies that is a gas under ambient conditions, absent nucleation sites.A “non-nucleated” material can include nucleating agent such as talc. A“polymeric material/blowing agent mixture” can be a single-phase,non-nucleated solution of at least the two, a nucleated solution of atleast the two, or a mixture in which blowing agent cells have grown.“Essentially closed-cell” microcellular material is meant to definematerial that, at a thickness of about 100 microns, contains noconnected cell pathway through the material. “Nucleating pathway” ismeant to define a pathway that forms part of microcellular polymericfoam extrusion apparatus and in which, under conditions in which theapparatus is designed to operate (typically at pressures of from about1500 to about 30,000 psi upstream of the nucleator and at flow rates ofgreater than about 10 pounds polymeric material per hour), the pressureof a single-phase solution of polymeric material admixed with blowingagent in the system drops below the saturation pressure for theparticular blowing agent concentration at a rate or rates facilitatingrapid nucleation. A nucleating pathway defines, optionally with othernucleating pathways, a nucleation or nucleating region of a device ofthe invention. “Reinforcing agent”, as used herein, refers to auxiliary,essentially solid material constructed and arranged to add dimensionalstability, or strength or toughness, to material. Such agents aretypified by fibrous material as described in U.S. Pat. Nos. 4,643,940and 4,426,470. “Reinforcing agent” does not, by definition, necessarilyinclude filler or other additives that are not constructed and arrangedto add dimensional stability. Those of ordinary skill in the art cantest an additive to determine whether it is a reinforcing agent inconnection with a particular material.

[0031] In preferred embodiments, the material of the invention ismicrocellular material and has average cell size of less than about 50microns. In some embodiments particularly small cell size is desired,and in these embodiments material of the invention has average cell sizeof less than about 30 microns, more preferably less than about 20microns, more preferably less than about 10 microns, and more preferablystill less than about 5 microns. The microcellular material preferablyhas a maximum cell size of about 100 microns or preferably less thanabout 75 microns. In embodiments where particularly small cell size isdesired, the material can have maximum cell size of about 50 microns,more preferably about 35 microns, and more preferably still about 25microns. A set of embodiments includes all combinations of these notedaverage cell sizes and maximum cell sizes. For example, one embodimentin this set of embodiments includes microcellular material having anaverage cell size of less than about 30 microns with a maximum cell sizeof about 50 microns, and as another example an average cell size of lessthan about 30 microns with a maximum cell size of about 35 microns, etc.That is, microcellular material designed for a variety of purposes canbe produced having a particular combination of average cell size and amaximum cell size preferable for that purpose. Control of cell size isdescribed in greater detail below.

[0032] Foam material of the invention has a void volume of at leastabout 5%, more preferably at least about 10%, more preferably at leastabout 15%, more preferably still at least about 20%, and more preferablystill at least about 30% according to one set of embodiments. These setof embodiments allow significant reduction in consumption of polymericmaterial. In another set of embodiments the material has a void volumeof at least about 50%, more preferably at least about 60%, morepreferably at least about 70%, and more preferably still at least about75%. Increasing cell density while maintaining essentially closed-cell,microcellular material where that material is desired can be achieved byusing high pressure drop rates as described in international patentapplication serial no. PCT/US97/15088, referenced above. Void volume, inthis context, means initial void volume, i.e, typically void volumeimmediately after extrusion and cooling to ambient conditions. That is,formation of foam material at a void volume of 50%, followed bycompaction resulting in a void volume of 40%, is still embraced by thedefinition of material at 50% void volume in accordance with theinvention.

[0033] The present invention provides systems and techniques forextrusion of standard-grade or nearly standard-grade polypropylene foam.Microcellular articles of the invention can be produced according to avariety of batch or continuous processes, such as those described inU.S. Pat. No. 5,158,986 of Cha, et al., U.S. patent application Ser. No.08/777,709, of Anderson, et al., filed Dec. 20, 1996 and entitled METHODAND APPARATUS FOR MICROCELLULAR POLYMER EXTRUSION, or InternationalPatent Application Serial No. PCT/US97/15088 of Anderson, et al., filedAug. 26, 1997, of the same title, each of which is incorporated hereinby reference.

[0034]FIGS. 1 and 2 describe extrusion systems that can be used inpolypropylene foaming according to the present invention. Referring toFIG. 1, an extrusion system 30 includes a barrel 32 having a first,upstream end 34 and a second, downstream end 36. Mounted for rotationwithin barrel 32 is an extrusion screw 38 operably connected, at itsupstream end, to a drive motor 40. Although not shown in detail,extrusion screw 38 includes feed, transition, gas injection, mixing, andmetering sections.

[0035] Positioned along extrusion barrel 32, optionally, are temperaturecontrol units 42. Control units 42 can be electrical heaters, caninclude passageways for temperature control fluid, or the like. Units 42can be used to heat a stream of pelletized or fluid polymeric materialwithin the extrusion barrel to facilitate melting, and/or to cool thestream to control viscosity, skin formation and, in some cases, blowingagent solubility. The temperature control units can operate differentlyat different locations along the barrel, that is, to heat at one or morelocations, and to cool at one or more different locations. Any number oftemperature control units can be provided.

[0036] Extrusion barrel 32 is constructed and arranged to receive aprecursor of polypropylene material. Typically, this involves a standardhopper 44 for containing pelletized polypropylene to be fed into theextruder barrel through orifice 46. Although preferred embodiments donot use chemical blowing agents, when chemical blowing agents are usedthey typically are compounded in polymer pellets introduced into hopper44.

[0037] Immediately downstream of the downstream end 48 of screw 38 inFIG. 1 is a region 50 which can be a temperature adjustment and controlregion, auxiliary mixing region, auxiliary pumping region, or the like.For example, region 50 can include temperature control units to adjustthe temperature of a fluid polymeric stream prior to nucleation, asdescribed below. Region 50 can include instead, or in addition, standardmixing units (not shown), or a flow-control unit such as a gear pump(not shown). In another embodiment, region 50 is replaced by a secondscrew of a tandem extrusion apparatus, the second screw optionallyincluding a cooling region.

[0038] Any of a wide variety of blowing agents can be used in connectionwith the present invention. Preferably, a physical blowing agent (ablowing agent that is a gas under ambient conditions) or mixture ofphysical blowing agents is used and, in this case, along barrel 32 ofsystem 30 is a port 54 in fluid communication with a source 56 of aphysical blowing agent. Physical blowing agents known to those ofordinary skill in the art such as hydrocarbons, chlorofluorocarbons,nitrogen, carbon dioxide, and the like can be used in connection withthis embodiment of the invention and, according to a preferredembodiment, source 56 provides an atmospheric blowing agent, mostpreferably carbon dioxide. A pressure and metering device 58 typicallyis provided between blowing agent source 56 and port 54. Supercriticalfluid blowing agents are especially preferred, in particularsupercritical carbon dioxide. Suitable chemical blowing agents includethose typically relatively low molecular weight organic compounds thatdecompose at a critical temperature or another condition achievable inextrusion and release a gas or gases such as nitrogen, carbon dioxide,or carbon monoxide. Examples include azo compounds such as azodicarbonamide. Where a chemical blowing agent is used, the blowingagents can be introduced into systems of a invention by being compoundedwithin polymer pellets feed into the system, or other techniquesavailable to those of ordinary skill in the art. Device 58 can be usedto meter the blowing agent so as to control the amount of the blowingagent in the polymeric stream within the extruder to maintain a level ofblowing agent at a level, according to one set of embodiments, betweenabout 1% and 15% by weight, preferably between about 3% and 12% byweight, more preferably between about 5% and 10% by weight, morepreferably still between about 7% and 9% by weight, based on the weightof the polymeric stream and blowing agent. In other embodiments it ispreferred that lower levels of blowing agent be used. As described inPCT/US97/15088, referenced above, different levels of blowing agent aredesirable under different conditions and/or for different purposes whichcan be selected in accordance with the invention.

[0039] The pressure and metering device can be connected to a controller(not shown) that also is connected to drive motor 40 and/or a drivemechanism of a gear pump (not shown) to control metering of blowingagent in relationship to flow of polymeric material to very preciselycontrol the weight percent blowing agent in the fluid polymeric mixture.

[0040] Although port 54 can be located at any of a variety of locationsalong the extruder barrel, according to a preferred embodiment it islocated just upstream from a mixing section 60 of the extrusion screwand at a location 62 of the screw where the screw includes unbrokenflights.

[0041] In a preferred embodiment of the blowing agent port system, twoports on opposing top and bottom sides of the barrel are provided. Inthis preferred embodiment, port 54 is located at a region upstream frommixing section of screw 38 (including highly-broken flights) at adistance upstream of the mixing section of no more than about 4 fullflights, preferably no more than about 2 full flights, or no more than 1full flight. Positioned as such, injected blowing agent is very rapidlyand evenly mixed into a fluid polymeric stream to quickly produce asingle-phase solution of the foamed material precursor and the blowingagent.

[0042] Port 54, in the preferred embodiment is a multi-hole portincluding a plurality of orifices connecting the blowing agent sourcewith the extruder barrel. In preferred embodiments a plurality of ports54 are provided about the extruder barrel at various positions radiallyand can be in alignment longitudinally with each other. For example, aplurality of ports 54 can be placed at the 12 o'clock, 3 o'clock, 6o'clock, and 9 o'clock positions about the extruder barrel, eachincluding multiple orifices. In this manner, where each orifice isconsidered a blowing agent orifice, the invention includes extrusionapparatus having at least about 10, preferably at least about 40, morepreferably at least about 100, more preferably at least about 300, morepreferably at least about 500, and more preferably still at least about700 blowing agent orifices in fluid communication with the extruderbarrel, fluidly connecting the barrel with a source of blowing agent.

[0043] Also in preferred embodiments is an arrangement in which theblowing agent orifice or orifices are positioned along the extruderbarrel at a location where, when a preferred screw is mounted in thebarrel, the orifice or orifices are adjacent full, unbroken flights. Inthis manner, as the screw rotates, each flight, passes, or “wipes” eachorifice periodically. This wiping increases rapid mixing of blowingagent and fluid foamed material precursor by, in one embodiment,essentially rapidly opening and closing each orifice by periodicallyblocking each orifice, when the flight is large enough relative to theorifice to completely block the orifice when in alignment therewith. Theresult is a distribution of relatively finely-divided, isolated regionsof blowing agent in the fluid polymeric material immediately uponinjection and prior to any mixing. In this arrangement, at a standardscrew revolution speed of about 30 rpm, each orifice is passed by aflight at a rate of at least about 0.5 passes per second, morepreferably at least about 1 pass per second, more preferably at leastabout 1.5 passes per second, and more preferably still at least about 2passes per second. In preferred embodiments, orifices are positioned ata distance of from about 15 to about 30 barrel diameters from thebeginning of the screw (at upstream end 34).

[0044] The described arrangement facilitates a method of the inventionthat is practiced according to one set of embodiments. The methodinvolves introducing, into fluid polymeric material flowing at a rate ofat least about 40 lbs/hr., a blowing agent that is a gas under ambientconditions and, in a period of less than about 1 minute, creating asingle-phase solution of the blowing agent fluid in the polymer. Theblowing agent fluid is present in the solution in an amount of at leastabout 2.5% by weight based on the weight of the solution in thisarrangement. In preferred embodiments, the rate of flow of the fluidpolymeric material is at least about 60 lbs/hr., more preferably atleast about 80 lbs/hr., and in a particularly preferred embodimentgreater than at least about 100 lbs/hr., and the blowing agent fluid isadded and a single-phase solution formed within one minute with blowingagent present in the solution in an amount of at least about 3% byweight, more preferably at least about 5% by weight, more preferably atleast about 7%, and more preferably still at least about 10% (although,as mentioned, in a another set of preferred embodiments lower levels ofblowing agent are used). In these arrangements, at least about 2.4 lbsper hour blowing agent, preferably CO₂, is introduced into the fluidstream and admixed therein to form a single-phase solution. The rate ofintroduction of blowing agent is matched with the rate of flow ofpolymer to achieve the optimum blowing agent concentration.

[0045] In the embodiment illustrated in FIG. 1, a system is providedhaving a multi-channel nucleator 66, including nucleating pathways,located substantially upstream of shaping die 68. As used herein,“nucleating pathway” is meant to define a pathway that forms part ofmicrocellular polymer foam extrusion apparatus and in which, underconditions in which the apparatus is designed to operate (typically atpressures of from about 1500 to about 30,000 psi upstream of thenucleator and at flow rates of greater than about 10 lbs polymericmaterial per hour), the pressure of a single-phase solution of polymericmaterial admixed with blowing agent in the system drops below thesaturation pressure for the particular blowing agent concentration at arate or rates facilitating nucleation. A nucleating pathway defines,optionally with other nucleating pathways, a nucleation or nucleatingregion of an extruder.

[0046] In one preferred embodiment, nucleator 66 has a polymer receivingend in fluid communication with the extrusion barrel, constructed andarranged to receive a fluid, non-nucleated, single-phase solution ofpolymeric material and blowing agent supplied by the barrel. Thenucleator includes a nucleated polymer releasing end in communicationwith residence chamber 70 constructed and arranged to contain nucleatedpolymeric material under conditions controlling cell growth, and a fluidpathway connecting the receiving end to the releasing end. Thearrangement allows for injecting blowing agent and maintaining the fluidstream, downstream of injection and upstream of nucleation, underpressure varying by no more than about 1,000 psi, preferably no morethan about 750 psi, and more preferably still no more than about 500psi. The fluid pathway of the nucleator has length and cross-sectionaldimensions that subject the single-phase solution, as a flowing stream,to conditions of solubility change sufficient to create sites ofnucleation at the microcellular scale in the absence of auxiliarynucleating agent. “At the microcellular scale” defines a cell densitythat, with controlled foaming, can lead to microcellular material. Whilenucleating agent can be used in some embodiments, in other embodimentsno new nucleating agent is used. In either case, the pathway isconstructed so as to be able to create sites of nucleation in theabsence of nucleating agent whether or not nucleating agent is present.In particular, the fluid pathway has dimensions creating a desiredpressure drop rate through the pathway. In one set of embodiments, thepressure drop rate is relatively high, and a wide range of pressure droprates are achievable. A pressure drop rate can be created, through thepathway, of at least about 0.1 GPa/sec in molten polymeric materialadmixed homogeneously with about 6 wt % CO₂ passing through the pathwayof a rate of about 40 pounds fluid per hour. Preferably, the dimensionscreate a pressure drop rate through the pathway of from about 0.2GPa/sec to about 1.5 GPa/sec, or from about 0.2 GPa/sec to about 1GPa/sec. The nucleator is constructed and arranged to subject theflowing stream to a pressure drop at a rate sufficient to create sitesof nucleation at a density of at least about 10⁷ sites/cm³. preferablyat least about 10⁸ sites/cm³.

[0047] The arrangement of FIG. 1, or a similar arrangement that involvesa single-channel nucleator located immediately upstream of shaping inassociation with a die, is constructed and arranged to continuouslynucleate a fluid stream of single-phase solution of polymeric materialand flowing agent flowing at a rate of at least 20 lbs/hour, preferablyat least about 40 lbs/hour, more preferably at least about 60 lbs/hour,more preferably at least about 80 lbs/hour, and more preferably still atleast about 100 lbs/hour. In FIG. 1 nucleation takes place significantlyupstream of shaping. In the working examples below, nucleation takesplace very closely upstream of final release and shaping. Anyarrangement can serve as a nucleator that subjects a flowing stream of asingle-phase solution of foamed material precursor and blowing agent toa solubility change sufficient to nucleate the blowing agent. Thissolubility change can involve a rapid temperature change, a rapidpressure change, for example caused by forcing material through anorifice where the rapid pressure drop takes place due to frictionbetween the material and the orifice wall, or a combination, and thoseof ordinary skill in the art will recognize a variety of arrangementsfor achieving nucleation in this manner. A rapid pressure drop to causenucleation is preferred. Where a rapid temperature change is selected toachieve nucleation, temperature control units can be provided aboutnucleator 66. Nucleation by temperature control is described in U.S.Pat. No. 5,158,986 (Cha., et al.) incorporated herein by reference.Temperature control units can be used alone or in combination with afluid pathway of nucleator 66 creating a high pressure drop rate influid polymeric material flowing therethrough.

[0048] The described arrangement allows for creation of a single-phasesolution at high flow rates. In particular, the arrangement allows forestablishing the stream of fluid polymeric material flowing in theextradite at a rate of at least 60 lbs/hour and introducing CO₂ blowingagent at a rate of at least 1 lb/hour into the stream at an injectionlocation to create a fluid stream including at least about 2.5% CO₂ byweight.

[0049] While creation of open-cell material is desirable for a varietyof products, closed-cell microcellular polypropylene is preferred in thepresent invention. To achieve rapid pressure drop to createmicrocellular material, while foaming controllably to maintainclosed-cell material, nucleating should be separated from shaping by adistance sufficient to achieve this control.

[0050] Also illustrated in FIG. 1 is an optional shaping element 69downstream of shaping die 68. Shaping element 69 can provide furthercontrol over the thickness or shape of an extruded product byrestricting expansion, further cooling the extradite (via, for example,fluid cooling channels or other temperature control units in element 69,not shown), or a combination. Without element 69, extradite is extrudedinto ambient conditions upon emergence from shaping die 68 (restrictedonly by polymeric extradite downstream of the exit of the shaping die).With element 69, the extradite generally emerges from shaping die intoconditions of pressure slightly above ambient.

[0051] With reference to FIG. 1, several arrangements of the inventionare described. In one, polymeric extradite emerges from a nucleatingpathway into ambient conditions and, where multi-channel nucleation isused, is recombined there. This would involve elimination of componentsdownstream of nucleator 66. In another arrangement, only forming element69 exists downstream of the nucleator. In another, the system includesnucleator 66, an enclosure downstream thereof (chamber 70) and aconstriction at the end of the chamber (forming die 68). In stillanother, the system includes nucleator 66, chamber 70, forming die 68,and forming element 69, as illustrated in the complete system of FIG. 1.Described another way, the invention includes one or more constrictionsconstructed and arranged to define nucleating pathway(s) and one or moreconstrictions upstream and/or downstream of the nucleating pathway(s)that each optionally include temperature control and/or shapingcapability. The system produces extruded article in the shape of acontinuous extrusion.

[0052] Referring now to FIG. 2, an alternate extrusion system 71 of theinvention is illustrated schematically, representative of the systemdescribed in the working examples below. System 71 includes a die 73similar to die 68 of FIG. 1, but including an exit 75 that is ofdimension creating a nucleating pathway. That is, a homogeneous,single-phase solution is created by the extruder in region 50 and, whenurged through nucleating pathway 75, the homogeneous, single-phasesolution is nucleated to form a nucleated fluid polymeric material whichthen is foamed and shaped optionally with the assistance of formingelement 69.

[0053] Polypropylene foams of the present invention can be blown with aphysical blowing agent such as carbon dioxide and thus, in preferredembodiments of the invention, the techniques of the invention do notrequire the added expense and complication of formulating a polymericprecursor to include a species that will react under extrusionconditions to form a blowing agent, especially the expense andcomplication of providing a copolymer component having chemicallyattached or grafted thereto a chemical blowing agent. Since foams blownwith chemical blowing agents inherently include residual, unreactedchemical blowing agent after a final foam product has been produced, aswell as chemical by-products of the reaction that forms a blowing agent,material of the present invention in this set of embodiments includesresidual chemical blowing agent or reaction by-product of chemicalblowing agent, in an amount less than that inherently found in articlesblown with 0.1% by weight chemical blowing agent or more, preferablyincluding residual chemical blowing agent or reaction by-product ofchemical blowing agent in an amount less than that inherently found inarticles blown with 0.05% by weight chemical blowing agent or more. Inparticularly preferred embodiments, the material is characterized bybeing essentially free of residual chemical blowing agent or free ofreaction by-products of chemical blowing agent. That is, they includeless residual chemical blowing agent or by-product than is inherentlyfound in articles blown with any chemical blowing agent.

[0054] One advantage of embodiments in which a chemical blowing agent isnot used or used in minute quantities is that recyclability of productis maximized. Use of a chemical blowing agent typically reduces theattractiveness of a polymer to recycling since residual chemical blowingagent and blowing agent by-products contribute to an overall non-uniformrecyclable material pool.

[0055] In one set of preferred embodiments, a polypropylene foam isprovided that has a unimodal molecular weight distribution. That is, theadded expense and complication of formulating compositions includingcopolymers, blends, or the like that have multi-modal molecular weightdistributions is not required. In particular, the foamed polymericarticle of the invention in this set of embodiments includes at leastabout 80% by weight polypropylene having a unimodal molecular weightdistribution. In more preferred embodiments the article includes atleast about 90% by weight polypropylene having a unimodal molecularweight distribution, more preferably about 95%, and more preferablystill the article consists essentially entirely or consists entirely ofunimodal molecular weight distribution polypropylene. The unimodal ormulti-modal characteristic of a polymer can be readily determined bythose of ordinary skill in the art using, for example, high temperaturegel permeation chromatography (GPC). For example, a Waters 150 CV GPCchromatograph may be used.

[0056] In another set of preferred embodiments the article is defined byfoamed polymeric material including at least about 80% by weighthomopolymeric polypropylene of viscosity of at least about 2.5×10³poise. This embodiment also avoids complication of significantcopolymerization or blending of auxiliary components. In preferredembodiments, at least about 90% by weight homopolymeric polypropylene ofviscosity of at least about 2.5×10³ poise is used, more preferably atleast about 95% by weight, and more preferably still the foamed articleconsists essentially entirely of or consists entirely of homopolymericpolypropylene of viscosity of at least about 2.5×10³ poise.

[0057] In accordance with each of these sets of preferred embodiments,the polypropylene article is preferably at least about 80% free ofcross-linking, more preferably at least about 90% free of cross-linking,or more preferably still essentially entirely free of cross-linking.

[0058] Preferred embodiments include all sets of combinations of theabove. For example, the article of the invention can include at leastabout 80% by weight homopolymeric polypropylene of viscosity of at leastabout 2.5×10³ poise that also is unimodal and is microcellular, or canconsist entirely of homopolymeric polypropylene having a unimodalmolecular weight distribution and a viscosity of at least about 2.5×10³poise and a maximum cell size of about 50 microns and an average cellsize of about 30 microns and being at least about 90% free ofcross-linking, etc.

[0059] Very thin product, such as sheet, can be made in accordance withthe invention, including tubes and other thin articles. According tothis aspect of the invention, microcellular material, preferablyessentially closed-cell material, of thickness less than about 4 mm,preferably less than about 3 mm, more preferably less than about 1 mm isproduced. In some embodiments extremely thin microcellular material isproduced, namely material of less than about 0.5 mm in thickness, morepreferably less than about 0.25 mm in thickness, more preferably stillless than about 0.2 mm in thickness. In some particularly preferredembodiments material on the order of 0.1 mm in thickness is produced.All of these embodiments can include essentially closed-cell material.

[0060] Thin product can be formed into a tubular configuration having,for example, a length-to-diameter ratio of at least 10 and wallthicknesses as described above. The length-to-diameter ratio can be atleast about 15 in preferred embodiments, more preferably at least about20, more preferably at least about 30, and more preferably still atleast about 50. In another embodiment the article is a tubular articlehaving a diameter-to-thickness ratio of from about 9:1 to about 50:1,more preferably from about 20:1 to about 40:1 and more preferably stillabout 30:1.

[0061] In a particularly preferred embodiment the present inventionfinds particular use in the fabrication of drinking straws. Those ofordinary skill in the art, and indeed the average consumer, willunderstand that drinking straws, used especially in connection withfast-food sales, must have at least the minimum rigidity,force/deflection value, and columnar strength without collapse, topuncture a perforated opening in a fast-food beverage cup lid and towithstand internal vacuum associated with drinking a relatively viscousbeverage such as a milk shake through a straw. It is a feature of thepresent invention that foam articles can both be fabricated at athickness of a drinking straw, and possesses the necessary physicalproperties for drinking straws according to these commercialrequirements. Accordingly, in the one aspect the invention provides afoam polymeric drinking straw. Use of a foam polymeric drinking strawboth reduces the raw material needed to produce the straw, and providesthe straw with opacity and white appearance that is desired in the caseof many drinking straws. One way to add opacity to a drinking straw isto add pigment. However, pigmented polymeric material is less amenableto recycling. The present invention provides thin, opaque, drinkingstraws that include less than about 1% by weight auxiliary opacifer,preferably less than about 0.05% by weight auxiliary opacifer, and morepreferably still material that is essentially free of auxiliaryopacifer. “Auxiliary opacifer”, in the present invention, is meant todefine pigments, dyes, or other species that are designed specificallyto absorb light, or talc or other materials that can block or diffractlight. Those of ordinary skill in the art can test whether an additiveis an opacifer. Microcellular straws of the invention have theappearance of essentially solid, white, plastic articles, which offerssignificant commercial appeal. In other embodiments, an opacifier can beadded.

[0062] Good toughness in the articles of the invention is achievedwithout necessity of reinforcing agents. Preferably, the articles of theinvention have less than about 10% reinforcing agent by weight, morepreferably less than about 5% reinforcing agent, more preferably stillless than about 2% reinforcing agent, and in particularly preferredembodiments the articles of the invention are essentially free ofreinforcing agent. “Reinforcing agent”, as used herein, refers toauxiliary, essentially solid material constructed and arranged to adddimensional stability, or strength or toughness, to material. Suchagents are typified by fibrous material as described in U.S. Pat. Nos.4,643,940 and 4,426,470. “Reinforcing agent” does not, by definition,include filler, colorant, or other additives that are not constructedand arranged to add dimensional stability. Since reinforcing agents areadded to increase dimensional stability, they typically are rod-like inshape or otherwise shaped to have a ratio, of a maximum dimension to aminimum dimension (length to diameter in the case of a rod or fiber) ofat least about 3, preferably at least about 5, more preferably at leastabout 10.

[0063] The function and advantage of these and other embodiments of thepresent invention will be more fully understood from the examples below.The following examples are intended to illustrate the benefits of thepresent invention, but do not exemplify the full scope of the invention.

EXAMPLE 1 Extrusion of Microcellular Homopolymer Polypropylene MaterialHaving a Fractional Melt Flow Rate

[0064] A tandem extrusion line (Akron Extruders, Canal Fulton, Ohio) wasarranged including a 2 inch, 32/1 L/D primary extruder and a 2.5 inch,34/1 L/D secondary extruder. An injection system for injection of CO₂into the primary was placed at a distance of approximately 20 diametersfrom the feed section. The injection system included 4 equally-spacedcircumferentially, radially-positioned ports, each port including 176orifices, each orifice of 0.02 inch diameter, for a total of 704orifices.

[0065] The primary extruder was equipped with a two-stage screwincluding conventional first-stage feed, transition, and meteringsections, followed by a multi-flighted (four flights) mixing section forblowing agent dispersion. The screw was designed for high-pressureinjection of blowing agent with minimized pressure drop between thefirst-stage metering section and point of blowing agent injection. Themixing section included 4 flights unbroken at the injection ports sothat the orifices were wiped (opened and closed) by the flights. At ascrew speed of 80 RPM each orifice was wiped by a flight at a frequencyof 5.3 wipes per second. The mixing section and injection system allowedfor very rapid establishment of a single-phase solution of blowing agentand polymeric material.

[0066] The injection system included air-actuated control valve toprecisely meter a mass flow rate of blowing agent at rates from 0.2 to12 lbs/hr at pressures up to 5500 psi.

[0067] The secondary extruder was equipped with a deep channel,three-flighted cooling screw with broken flights, which provided theability to maintain a pressure profile of microcellular materialprecursor, between injection of blowing agent and entrance to the pointof nucleation (the die, in this case) varying by no more than about 1500psi, and in most cases considerably less.

[0068] The system included instrumentation allowing measurement ofpressure and temperature of the melt stream at least six locationsthroughout the tandem system between a location just prior to theblowing agent injection ports to the point of entry into the die toprecisely monitor material conditions. Along the screw, melt temperaturewas measured with infrared equipment to avoid disruption of the meltstream.

[0069] PP pellets were gravity-fed from a hopper into the extrusionsystem. The grade used was a standard homopolymer resin (Montell 6823),having a nominal melt flow index of 0.5 g/10 min. Primary screw speedwas 40 RPM, giving a total output of approximately 36 lbs/hr ofmaterial. Secondary screw speed was 13 RPM. Barrel temperatures of thesecondary extruder were set to maintain a melt temperature of 380° F.measured at the end of the secondary extruder. CO₂ blowing agent wasinjected at a rate of 1.8 lbs/hr resulting in 5.0% blowing agent in themelt. Pressure profile between the injection ports and the inlet of thedie was maintained between 2350 and 2780 psi. The die placed at the endof the secondary extruder was a circular orifice of constant crosssection with a diameter of 0.080 inches and a length of 1.2 inches. Thepressure drop rate across the die was approximately 1.1 GPa/s.

[0070]FIG. 3 is a photocopy of an SEM image of the cross section of theextrudate, showing uniform, spherical, relatively closed cells, themajority of which were from 20 to 50 microns in diameter. Materialdensity was approximately 0.35 g/cm³ (19.3 lbs/ft³), and cell densitywas approximately 3.7×10⁷ cells/cm³.

EXAMPLE 2 Extrusion of Microcellular Homopolymer Polypropylene MaterialHaving Talc Filler

[0071] System and parameters were used as in Example 1. PP pellets weregravity-fed from a hopper into the extrusion system. The grade used wasa standard talc-filled homopolymer resin (Montell Astryn 65F4-4). Thebase resin had a nominal melt flow index of 4 g/10 min., and was filledwith 40% by weight of talc. Primary screw speed was 60 RPM, giving atotal output of approximately 63 lbs/hr of material. Secondary screwspeed was 24 RPM. Barrel temperatures of the secondary extruder were setto maintain a melt temperature of 326° F. measured at the end of thesecondary extruder. CO₂ blowing agent was injected at a rate of 2.0lbs/hr resulting in 5.3% blowing agent in the melt. Pressure profilebetween the injection ports and the inlet of the die was maintainedbetween 2560 psi and 3250 psi. The die placed at the end of thesecondary extruder was a circular orifice of constant cross section witha diameter of 0.080 inches and a length of 1.2 inches. The pressure droprate across the die was 1.7 GPa/s.

[0072]FIG. 4 is a photocopy of an SEM image of the cross section of theextrudate, showing uniform, partially closed cells of an average ofabout 10 microns diameter, with a maximum size of about 30 micronsdiameter. Material density was approximately 0.61 g/cm3 (38 lbs/ft3),and cell density was approximately 1.0×109 cells/cm3.

EXAMPLE 3 Extrusion of Microcellular Homopolymer Polypropylene LowDensity Sheet

[0073] System and parameters were used as in Example 1. PP pellets weregravity-fed from a hopper into the extrusion system. The grade used wasa standard homopolymer resin (Montell 6823), having a nominal melt flowindex of 0.5 g/10 min. Primary screw speed was 80 RPM, giving a totaloutput of approximately 65 lbs/hr of material. Secondary screw speed was20 RPM. Barrel temperatures of the secondary extruder were set tomaintain a melt temperature of 328° F. measured at the end of thesecondary extruder. CO₂ blowing agent was injected at a rate of 5.2lbs/hr resulting in 8.0% blowing agent in the melt. Pressure profilebetween the injection ports and the inlet of the die was maintainedbetween 2630 psi and 3880 psi. The system included, at the end of thesecondary extruder, a die adapter and a cylindrical annular die with anexit gap of 0.053 inches with a diameter of 1.25 inches diverging from anucleation gap of 0.022 thickness, 0.563 inch length, and diameter of0.853 inch. The die adapter was equipped with taps for measurement ofmelt temperature and pressure just prior to entry into the die. Thepressure drop rate across the die was 0.4 GPa/s.

[0074]FIG. 5 is a photocopy of an SEM image of the cross section of theextrudate, showing nominally hexagonal-shaped thin wall cells at leastpartially closed in structure. Average cell size was about 70 microns,with maximum size of 100 microns diameter. Material density was measuredto be 0.13 g/cm³ (8.1 lbs/ft³), and cell density was approximately1.7×10⁷ cells/cm³.

EXAMPLE 4 Microcellular Polypropylene Talc Filled Tubular Product

[0075] An NRM (Pawcatuck, Conn.) 2.5 inch 44/1 L/D long single extrusionline was equipped with an injection system for injection of CO₂ placedat a distance of approximately 25 diameters from the feed section. Theinjection system included 4 equally-spaced circumferentially,radially-positioned ports, each port including 417 orifices, eachorifice of 0.02 inch diameter, for a total of 1668 orifices.

[0076] The extruder was equipped with a two-stage screw includingconventional first-stage feed, barrier flight transition, and meteringsections, followed by a multi-flighted (six flights) mixing section forblowing agent dispersion. The screw was designed for high-pressureinjection of blowing agent with minimized pressure drop between thefirst-stage metering section and point of blowing agent injection. Thesecond stage of the screw included a mixing section having 6 flightsunbroken at the injection ports so that the orifices were wiped (openedand closed) by the flights. At a screw speed of 80 RPM each orifice waswiped by a flight at a frequency of 8 wipes per second. The mixingsection and injection system allowed for very rapid establishment of asingle-phase solution of blowing agent and polymeric material. Theinjection system included an air-actuated control valve to preciselymeter a mass flow rate of blowing agent at rates from 0.2 to 50 lbs/hrat pressures up to 5500 psi.

[0077] The second stage of the screw was also equipped with a deepchannel, three-flighted cooling section with broken flights, whichprovided the ability to cool the polymer melt stream.

[0078] The system included, at the end of the extruder, a die adapterand a cylindrical annular die with a gap of 0.34 inch, inner diameter of0.4 inch, and land length of 2 inches. The die adapter was equipped withtaps for measurement of melt temperature and pressure just prior toentry into the die.

[0079] The system included instrumentation allowing measurement ofpressure and temperature of the melt stream at least 7 locationsthroughout the system between a location just prior to the blowing agentinjection ports to the point of entry into the die to precisely monitormaterial conditions. Along the screw, melt temperature was measured withinfrared equipment to avoid disruption of the melt stream.

[0080] A standard homopolymer polypropylene resin (Solvay HB 1301),having a nominal melt flow index of 5 g/10 min., was used as the baseresin. Talc concentrate consisting of pellets having 40 percent byweight of talc dispersed in a homopolymer polypropylene matrix wereblended with the HB 1301 using a loss-in-weight type blending system toproduce a mixture containing 5% by weight of talc. This mixture was thengravity fed from a hopper into the extrusion system. Primary screw speedwas 50 RPM, giving a total output of approximately 54 lbs/hr ofmaterial. Barrel temperatures were set to maintain a melt temperature of422° F. measured at the end of the extruder. CO₂ blowing agent wasinjected at a rate of 0.3 lbs/hr resulting in 0.5 5% blowing agent inthe melt. A die adapter was attached to the discharge of the extruder,connecting to a cylindrical annular die having a gap of 0.025 incheswith an outer diameter of 0.118 inches and a land length of 0.15 inches.Pressure profile between the injection ports and the inlet of the diewas maintained between 2430 and 3540 psi. The pressure drop rate acrossthe die was 11.2 GPa/s.

[0081]FIG. 6 is a photocopy of an SEM image of the cross section of theextrudate, showing generally spherical cells approximately 50 microns indiameter dispersed throughout the cross section of the tube wall. Wallthickness of the product was approximately 0.008 inch (0.21 mm). Productouter diameter was about 0.26 inch (6.60 mm). Material density wasapproximately 0.51 g/cm³ (32 lbs/ft³), and cell density wasapproximately 2.0×10⁷ cells/cm³.

EXAMPLE 5 Microcellular Polypropylene Tubular Product with Talc andColor Concentrate

[0082] System and parameters were used as in Example 4. A resinformulation having the same base PP resin grade as Example 4, but with4% of TiO₂ pellet color concentrate and 3% talc was blended and gravityfed from a hopper into the extrusion system. Primary screw speed was 50RPM, giving a total output of approximately 54 lbs/hr of material.Barrel temperatures were set to maintain a melt temperature of 404° F.measured at the end of the extruder. CO₂ blowing agent was injected at arate of 0.3 lbs/hr resulting in 0.55% blowing agent in the melt. A dieadapter was attached to the discharge of the extruder, connecting to acylindrical annular die having a gap of 0.025 inches with an outerdiameter of 0.18 inches and a land length of 0.15 inches. Pressureprofile between the injection ports and the inlet of the die wasmaintained between 2710 and 3950 psi. The pressure drop rate across thedie was 12.6 GPa/s.

[0083]FIG. 7 is a photocopy of an SEM image of the cross section of theextrudate, showing generally spherical cells approximately 35 microns indiameter dispersed throughout the cross section of the tube wall. Wallthickness of the product was approximately 0.007 inch (0.18 mm). Productouter diameter was about 0.26 inch (6.60 mm). Material density wasapproximately 0.57 g/cm³ (36 lbs/ft³), and cell density wasapproximately 5.2×10⁶ cells/cm³.

EXAMPLE 6 Microcellular Fractional Melt Flow Polypropylene MediumDensity Sheet

[0084] System and parameters were used as in Example 1, but with a 2.5inch primary extruder and a 3 inch secondary extruder. PP pellets weregravity-fed from a hopper into the extrusion system. The grade used wasa standard homopolymer resin (Montell 6823), having a nominal melt flowindex of 0.5 g/10 min. Primary screw speed was 90 RPM, giving a totaloutput of approximately 84 lbs/hr of material. Secondary screw speed was5 RPM. Barrel temperatures of the secondary extruder were set tomaintain a melt temperature of 386° F. measured at the end of thesecondary extruder. CO₂ blowing agent was injected at a rate of 4.0lbs/hr resulting in 4.8% blowing agent in the melt. A die adapter at thedischarge of the secondary extruder was connected to a flat sheet T-typedie having a die exit of 4.5 inches width and gap of 0.034 inch. Aseparate nucleator of constantly decreasing gap to an exit dimension of0.015 inch was positioned within 0.5 inches of the die exit. The die hadboth melt and pressure indicators. Pressure profile between theinjeciton ports and the inlet of the die was maintainted between 2120and 3490 psi. The overall pressure drop rate across the die lips was0.07 GPa/s.

[0085]FIG. 8 is a photocopy of an SEM image of the cross section of theextrudate, showing a uniform dispersion of cells having an averagediameter of about 25 microns. Material density was approximately 0.63g/cm³ (39 lbs/ft³, and cell density was approximately 2.7×10⁷ cells/cm³.

EXAMPLE 7 Microcellular Polypropylene Medium Density Sheet

[0086] The extrusion system was identical to that of Example 6, with theexception of the die lip dimensions. PP pellets were gravity-fed from ahopper into the extrusion system. The grade used was a standardhomopolymer resin (Solvay HB 3052), having a nominal melt flow index of1.5 g/10 min. Primary screw speed was 50 RPM, giving a total output ofapproximately 100 lbs/hr of material. Secondary screw speed was 13 RPM.Barrel temperatures of the secondary extruder were set to maintain amelt temperature of 379° F. measured at the end of the secondaryextruder. CO₂ blowing agent was injected at a rate of 5.5 lbs/hrresulting in 5.5% blowing agent in the melt. The die exit width was 10inches, with a constant 5° included angle tapered exit of 0.5 inchlength having and exit gap of 0.007 inch. Pressure profile between theinjection ports and the inlet of the die was maintainted between 1960and 2210 psi. The overall pressure drop rate across the die lips was0.06 GPa/s.

[0087]FIG. 9 is a photocopy of an SEM image of the cross section of theextrudate, showing small unifrom, homogeneously distributed cells ofaverage diameter of approximately 10 microns. Material density wasapproximately 0.57 g/cm³ (36 lbs/ft³), and cell density wasapproximately 5.8×10⁸ cells/cm³.

[0088] Those skilled in the art would readily appreciate that allparameters listed herein are meant to be exemplary and that actualparameters will depend upon the specific application for which themethods and apparatus of the present invention are used. It is,therefore, to be understood that the foregoing embodiments are presentedby way of example only and that, within the scope of the appended claimsand equivalents thereto, the invention may be practiced otherwise thanas specifically described.

What is claimed is:
 1. An article comprising foamed microcellularpolypropylene having an average cell size less than about 100 microns.2. An article as in claim 1 , the article including at least about 80%by weight polypropylene having a unimodal molecular weight distribution.3. An article as in claim 1 , the article including at least about 90%by weight polypropylene having a unimodal molecular weight distribution.4. An article as in claim 1 , the article including at least about 95%by weight polypropylene having a unimodal molecular weight distribution.5. An article as in claim 1 , consisting essentially of polypropylenehaving a unimodal molecular weight distribution.
 6. An article as inclaim 1 , consisting of polypropylene having a unimodal molecular weightdistribution.
 7. An article as in claim 1 , comprising foamed polymericmaterial including at least about 80% by weight homopolymericpolypropylene of viscosity of at least about 2.5×10³ poise.
 8. Anarticle as in claim 1 , comprising foamed polymeric material includingat least about 90% by weight homopolymeric polypropylene of viscosity ofat least about 2.5×10³ poise.
 9. An article as in claim 1 , comprisingfoamed polymeric material including at least about 95% by weighthomopolymeric polypropylene of viscosity of at least about 2.5×10³poise.
 10. An article as in claim 1 , comprising foamed polymericmaterial consisting essentially of homopolymeric polypropylene ofviscosity of at least about 2.5×10³ poise.
 11. An article as in claim 1, comprising foamed polymeric material consisting of homopolymericpolypropylene of viscosity of at least about 2.5×10³ poise.
 12. Anarticle as in claim 1 , formed as a drinking straw.
 13. An article as inclaim 1 , having a length-to-diameter ratio of at least 10, a wallthickness of no more than 1.0 mm.
 14. An article as in claim 1 ,comprising a foamed polymeric tubular article having adiameter-to-thickness ratio of from about 9:1 to about 50:1.
 15. Anarticle as in claim 1 , consisting essentially of homopolymericpolypropylene having a unimodal molecular weight distribution and havinga viscosity of at least about 2.5×10³ poise.
 16. An article comprisingfoamed polymeric material including at least about 80% by weightpolypropylene having a unimodal molecular weight distribution.
 17. Anarticle as in claim 16 , comprising foamed polymeric material includingat least about 90% by weight polypropylene having a unimodal molecularweight distribution.
 18. An article as in claim 16 , comprising foamedpolymeric material including at least about 95% by weight polypropylenehaving a unimodal molecular weight distribution.
 19. An article as inclaim 16 , comprising foamed polymeric material consisting essentiallyof polypropylene having a unimodal molecular weight distribution.
 20. Anarticle as in claim 16 , comprising foamed polymeric material consistingof polypropylene having a unimodal molecular weight distribution.
 21. Anarticle as in claim 16 , formed as a drinking straw.
 22. An article asin claim 16 , having a length-to-diameter ratio of at least 10, a wallthickness of no more than 1.0 mm.
 23. An article as in claim 16 ,comprising a foamed polymeric tubular article having adiameter-to-thickness ratio of from about 9:1 to about 50:1.
 24. Anarticle comprising foamed polymeric material including at least about80% by weight homopolymeric polypropylene of viscosity of at least about2.5×10³ poise.
 25. An article as in claim 24 , comprising the articleincluding at least about 90% by weight homopolymeric polypropylene ofviscosity of at least about 2.5×10³ poise.
 26. An article as in claim 24, comprising the article including at least about 95% by weighthomopolymeric polypropylene of viscosity of at least about 2.5×10³poise.
 27. An article comprising foamed polymeric material consistingessentially of homopolymeric polypropylene of viscosity of at leastabout 2.5×10³ poise.
 28. An article comprising foamed polymeric materialconsisting of homopolymeric polypropylene of viscosity of at least about2.5×10³ poise.
 29. An article as in claim 24 , comprising foamedpolymeric material including at least about 90% by weight homopolymericpolypropylene of viscosity of at least about 2.5×10³ poise.
 30. Anarticle as in claim 29 , having a length-to-diameter ratio of at least10, a wall thickness of no more than 1.0 mm.
 31. An article as in claim29 , comprising a foamed polymeric tubular article having adiameter-to-thickness ratio of from about 9:1 to about 50:1.
 32. Anarticle comprising polymeric material including at least about 80% byweight polypropylene having a unimodal molecular weight distribution andviscosity of at least about 2.5×10³ poise.
 33. An article as in claim 32, comprising microcellular material.
 34. An article as in claim 33 ,consisting essentially of microcellular polypropylene having a unimodalmolecular weight distribution and a viscosity of at least about 2.5×10³poise.
 35. An article comprising a foam polymeric drinking straw.
 36. Anarticle as in claim 35 , wherein the foam polymeric drinking strawcomprises microcellular material.
 37. An article as in claim 36 ,wherein the polymeric drinking straw includes at least about 80% byweight polypropylene having a unimodal molecular weight distribution.38. An article as in claim 36 , wherein the polymeric drinking strawincludes at least about 80% by weight homopolymeric polypropylene ofviscosity of at least about 2.5×10³ poise.
 39. An article as in claim 38, wherein the drinking straw includes at least about 80% by weightpolypropylene having a unimodal molecular weight distribution.
 40. Anarticle as in claim 35 , the article comprising a microcellularpolymeric drinking straw consisting essentially of homopolymericpolypropylene having a unimodal molecular weight distribution and havinga viscosity of at least about 2.5×10³ poise.
 41. An article comprising afoamed polymeric tubular article having a length-to-diameter ratio of atleast 10 and a wall thickness of no more than about 1.0 mm.
 42. Anarticle comprising a foamed polymeric tubular article having adiameter-to-thickness ratio of from about 9:1 to about 50:1.